1. Field of the Invention
The present invention relates to a digital servo apparatus for optical disc players and, more particularly, to a track traverse detecting circuit contained in the digital servo apparatus to detect the traverse operation of an optical pickup spot across tracks (i.e., pit arrays) on an optical disc.
2. Description of the Prior Art
When starting to play back a disc, optical disc players such as CD players read and store beforehand TOC (table of contents) data from a read-in area of the currently set disc. Upon access to a desired track based on a search command, the microcomputer in the optical disc player first calculates the currently read sub-code Q data to find the current track number and then the number of the desired track. The difference between the two track numbers allows the microcomputer to find the number of tracks to be jumped. Accordingly the microcomputer drives the optical system of the player to jump the calculated number of tracks in order to reach the target track. The need to count the number of tracks jumped upon jump operation requires detecting the traverse operation of the optical pickup spot across tracks. FIG. 1 is a block diagram of a prior art tracking servo system for controlling the jump operation. In FIG. 1, the information recorded on a disc 1 is read by an optical pickup 2. The read RF signal is sent to a demodulating system, not shown, via an RF amplifier 3 as well as to a track traverse detecting circuit 4.
A tracking error signal generating circuit 5 generates a tracking error (TE) signal based on the output of the pickup 2. Generated by the known three-beam generation method or the like, the TE signal has a S-curve characteristic. The TE signal is then fed to the track traverse detecting circuit 4. In turn, the track traverse detecting circuit 4 generates a count-out signal Cout based on the RF signal and TE signal. The count-out signal Cout is a signal that indicates the traverse operation of the optical pickup spot across a track. The signal Cout is supplied to .mu.CPU (microcomputer) 6.
The .mu.CPU 6 counts the count-out signal Cout that is generated every time the optical pickup spot traverses a track. When the count value coincides with the track count held beforehand in a register in keeping with the search target address, the .mu.CPU 6 issues a jump stop command to a tracking controller 7. Accordingly the tracking controller 7 operates a tracking actuator, not shown, in the pickup 2 through an actuator driver 8.
FIG. 2 is a clock diagram of the typical prior art track traverse detecting circuit 4. In FIG. 2, the RF signal and TE signal are digitized by an A/D converter 11 before they are sent to a mirror signal (mirror face detection signal) generating circuit 12 and a TZC (tracking zero cross) signal generating circuit 13. The mirror signal generating circuit 12 generates a high-level mirror signal ( a ) ( see FIG. 3) by detecting a mirror face between tracks (pit arrays) on the disc 1.
The mirror signal (a) is a signal which goes High when the optical spot is irradiated at a disc mirror face (area between tracks or pit arrays), and which goes Low when the optical spot is irradiated at a track. A TZC signal (c) is generated when the sign bit of the TE signal (b) is captured, the TE signal representing the deviation of the optical spot from any of the tracks or from any mirror area therebetween. When the TE signal (b) is zero, i.e., when the optical spot is located precisely on a track or on a mirror face, the TZC signal (c) goes from High to Low or vice versa.
During a traverse operation, a TZC edge signal (d) is generated at the time when a midpoint is reached between a High and a Low period of the mirror signal. This is how the phase difference becomes substantially 90 degrees between mirror signal (a) and TZC signal (c). In a digitized setup, however, the delay resulting from the sampling by the A/D converter and the delay stemming from a noise-reducing LPF (low pass filter ) combine to delay the mirror signal (a) in phase.
The TZC signal generating circuit 13 generates the TZC signal (c) by detecting the zero cross timing of the TE signal (b). The TZC (c) signal is supplied to an edge detecting circuit 14. The edge detecting circuit 14 detects leading and trailing edges of the TZC signal (c). The TZC edge signal (d) output by the edge detecting circuit 14 is fed as a sampling signal to a sampling circuit 15 for sampling of the mirror signal (a). The output of the sampling circuit 15 is provided as the count-out signal Cout (e) which is a track traverse signal.
The growing use of digital servo circuits today requires the mirror signal (a) to be generated by a mirror signal generating circuit 12 of a digital circuit construction, as described. In such cases, the generation timing of the mirror signal (a) is delayed illustratively by two factors, the first being the delay from the sampling of the A/D converter 11, the other being the delay due to a noise-reducing LPF (low pass filter) in the mirror signal generating circuit 12. Theoretically, there should be the 90-degree phase difference between mirror signal (a) and TZC signal (c), as shown in FIG. 3.
For the reasons described, the generation timing of the mirror signal (a) tends to be delayed. This poses no serious problem when the traverse speed is not very high during track search. However, as the traverse speed is increased, a delayed-mirror signal (a) develops a disturbed phase relationship relative to the TZC signal (c). When the traverse speed is very high, the TZC signal (c) can overtake the mirror signal (a) in phase.
At low-speed traverse time, the delay of the mirror signal (a) is negligible with respect to its pulse width, as shown in FIG. 3. As the traverse speed is increased, the pulse width of the mirror signal (a) is reduced. This raises the delay ratio of the mirror signal (a) with respect to its pulse width. In turn, the phase difference becomes smaller than the theoretical 90 degrees between mirror signal (a) and TZC signal (c).
The signal Cout is obtained by sampling the mirror signal (a) using the TZC edge signal (d). Since what needs to be accomplished here is for the microprocessor to count the number of tracks traversed, there is no intrinsic need for the 90 degree phase difference to be maintained between mirror signal (a) and TZC signal (c). However, as the traverse speed is made still higher, with the result that edges of the mirror signal are delayed relative to the TZC signal (c), the signal Cout tends to develop missing edges. This means that the ,microprocessor is now incapable of accurately counting the number of traversed tracks.
At high-speed traverse time in particular, the count-out signal Cout is most likely to develop missing edges, as illustrated by the signal Cout at 16, 17, 18 and 19 FIG. 5. This phenomenon, making it impossible to count the number of traversed tracks accurately, exemplifies the inability of the prior art to acquire precise track counts. This is a major impediment to the accurate provision of track-related control functions including track indexing.